1. Field of the Invention
The present invention relates to an inkjet printer and inkjet printing method, in particular, it relates to an inkjet printer and inkjet printing method for printing small droplet of ink at a high density and high frequency.
2. Description of the Related Art
Small droplet, high density nozzles and high driving frequencies have been promoted in inkjet printers. Under such circumstances, there has recently arisen a new problem called “end-deviation.”
FIG. 1 is a schematic view showing an “end-deviation.” In FIG. 1, the reference numeral 11 denotes a printing head, and the printing head 11 vertically moves while ejecting ink droplets 13 from a plurality of ejection ports arranged at an ejection port surface 14 at a high density. The ejected ink droplets 13 impact a print medium 12 to form a dot. In a high ejecting frequency of the printing head, air with viscosity surrounding the ink droplets 13 move with a movement of the ink droplet 13 flying toward the print medium 12 at a high density. As a result, a pressure in the vicinity of the ejection port surface 14 becomes smaller than that of the periphery of the printing head 11, and air surrounding the above air flows into the decompressed area in a direction shown by the arrows. The airflow especially deflects the ink droplets 13 ejected from the ejection ports positioned at both ends of an ejection port array toward the ejection ports positioned at the center thereof, and makes the ink droplets 13 impact a position deviated from a target position on the print medium 12.
FIG. 2 is a graph showing test results that the inventors performed to check the degree of the above “end-deviation.” In this case, the distance (distance to the paper) from the ejection port surface 14 to the print medium 12 was 1.3 mm, 128 ejection ports were arranged at intervals of approximately 21.2 μm, the ejection volume from each ejection port was 2.8 pl, and the ejecting frequency from each ejection port was 25 KHz. In FIG. 2, the horizontal axis indicates each arrangement position of the aligned ejection ports. In addition, the vertical axis indicates a deviation amount of a position, where the ink droplets ejected from each ejection port actually impact, from the target position. Here, in the state shown in FIG. 1, the case of impacting from the right side of the target position is shown as “+,” and the case of impacting from the left side is shown as “−.” That is, FIG. 2 reveals that the ink droplets ejected from the ejection ports at the outermost both ends are deviated to innermost sides and printed (approximately 10 μm), the deviation amount is slowly reduced as the position of the ejection port becomes close to the center, and that the print position deviation amount of the ink droplets ejected from the center ejection port becomes smallest.
FIG. 3 is a view showing a print state in the case of actually printing a uniform image with the printing head which generates such a print state. The printing head 11 mounted on a carriage moves from left to right in FIG. 3 at a predetermined speed while ejecting ink from each ejection port 31 at a fixed ejecting frequency. An image 32 formed by a first print scanning and an image 33 formed in a second print scanning are shown in FIG. 3. The ink droplets ejected from the ejection ports at the end of the printing head are deflected toward the center of the printing head to impact the print medium, and thus an area to be naturally printed by the ink droplets ejected from the ejection ports at the end appears as a blank area 34. Such a blank area 34 is generated at each connecting part between the print scannings to lower the quality of a uniform image area.
The “end-deviation” is generally easily checked as the ejection volume becomes small, the ejecting frequency is high and the arrangement density of the ejection ports is high, in particular, it becomes apparent when the ejection volume is not more than 10 pl.
FIG. 4 is a graph showing a relationship between the ejection volume and an end-deviation amount examined by the inventors. Here, a printing head having the same conditions as the printing head shown in FIG. 1 is used, the horizontal axis shows variation of the ejection volume from approximately 5 pl to 16 pl, and the vertical axis shows the print position deviation amount of the ink droplets ejected from the ejection ports positioned at the outermost end. FIG. 4 reveals that the print position deviation amount becomes large as the ejection volume becomes small. Therefore, it is considered that, as the ink droplets become small, the rate of the surface area to weight of the ink droplets is increased and the ink droplets easily receive influence from the airflow.
Regarding the “end-deviation” as described above, various countermeasures have been proposed. For example, a constitution is disclosed that an amount of the ink droplets ejected from the ejection ports positioned at the further end is set large in advance in arranging a plurality of ejection ports in the printing head. Thus, an inertia weight of the ink droplets from the end can be increased, and the airflow in the vicinity of the end hardly has influence on the ink droplets. However, making the ink droplets lager prevents an image having high definition and high gradation from being formed. In the inkjet printer having the advantage of performing printing with small droplets of ink at a high definition, increasing the ejection volume although partially is not suitable as a solving method of the end-deviation.
On the other hand, Japanese Published Unexamined Patent Application No. 2003-145775 discloses a printing head in which ejection ports positioned at the end are arranged at pitches larger than that of the center. For example, ejection ports positioned in the vicinity of the end are arranged at larger intervals than 21.2 μm in anticipation of the print position deviation, so that dots are printed on the print medium at pitches of 21.2 μm even if the end-deviation arises. However, even when a low duty image, in which the end deviation can hardly arise, is formed with the printing head having such a constitution, a print position of the ink droplets ejected from the ejection ports positioned at the end is deviated further outward. Accordingly, in this case, an area arises in which the connecting part between the print scans excessively overlap with each other, and there is a risk that black streaks become conspicuous.
The “end-deviation” arises under conditions with a small ejection volume, a high ejecting frequency, and a high print density. Accordingly, if any one of the conditions is removed, the “end-deviation” can be reduced. However, these conditions are all essential for printing an image having a high resolution and high quality at a high speed. Accordingly, a reducing method of the “end-deviation” without removing the conditions is demanded.
Japanese Patent Laid-Open Nos. 2002-096455 and 2002-292910 disclose a reducing method of the “end-deviation” adverse effects by providing a mask pattern to be used in performing a multi-pass printing method with features.
FIG. 5 is an explanatory schematic view of the multi-pass printing method. Here, a two-pass type multi-pass printing method is shown which completes an image in an arbitrary area by two print scannings. In FIG. 5, the reference numeral 1200 denotes a printing head having ejection port arrays for four colors. The printing head 1200 ejects ink droplets while moving in a main scanning direction in FIG. 5 to print dots onto the print medium.
However, in the multi-pass printing method, printing is not performed for all printable pixels by only one print scanning. For example, in the two-pass type multi-pass printing, printing is performed for approximately half of all printable pixels via the ejection ports positioned at the lower half part of the printing head 1200 in a first print scanning. When the first print scanning is performed, the print medium is conveyed by a length corresponding to half of a print width of the printing head 1200 in a sub-scanning direction in FIG. 5.
In the following second print scanning, printing is performed for the remaining pixels via the ejection ports positioned at the upper half part of the printing head 1200 in the image area where the printing has already been performed for approximately half of all pixels by the first print scanning. In addition, in the second print scanning, the lower half part of the printing head 1200 performs printing for the pixels of approximately half of the blank area adjacent to the image area. When the second print scanning ends, the print medium is further conveyed by the length corresponding to half of the print width of the printing head 1200 in the sub-scanning direction in FIG. 5.
In the two-pass type multi-pass printing method, the image is formed in stages by alternately repeating the above print main scanning to half of all pixels and the sub-scanning of the length corresponding to half of the print width. According to the multi-pass printing method, the image is formed in the identical image area on the print medium by a plurality of print scannings via the ejection port groups different from each other in the printing head. Accordingly, even if there are variations in the ejecting direction and the ejection volume of the ejection ports, and even if there are some variations in conveying amount of the print medium, it is possible to make adverse effects due to the variations inconspicuous. Furthermore, although the two-pass type multi-pass printing method for completing an image by the two print main scannings is described above with reference to FIG. 5, the number of multi-passes is not limited thereto. As the number of multi-passes is increased, a formed image becomes excellent in uniformity.
When the above-described multi-pass printing method is employed, a mask pattern, in which permission or non-permission of printing is determined, is frequently used in order to determine pixels for which the printing is to be performed by each print main scanning. Various image quality items other than uniformity can be improved by providing such a mask pattern with various features.
FIG. 6 is disclosed in Japanese Patent Laid-Open No. 2002-096455, and is a view showing mask patterns which are improved to avoid the end-deviation. Here, a printing head having 768 ejection ports is employed, and mask patterns used for performing four-pass type multi-pass printing is shown. The size of the mask pattern is 768 areas corresponding to the number of ejection ports in a vertical direction, and 256 areas in a horizontal direction. An area shown by black is a print permission pixel, and an area shown by white is a print non-permission pixel. The print permission or print non-permission of each pixel is determined so that the four mask patterns corresponding to four ejection port groups respectively are complementary to each other.
As shown in FIG. 6, a bias is provided between the numbers of print permission pixels in accordance with positions of the ejection ports. A print permission rate of the ejection port at the end is lowered compared with that of the center so that adverse effects due to impact position deviations of the ink droplets ejected from the ejection ports at the end can be made inconspicuous.
Japanese Patent Laid-Open No. 2002-96455 discloses that a bias is provided between the numbers of print permission pixels in accordance with positions of ejection ports. Furthermore, Japanese Patent Laid-Open No. 2002-96455 discloses that it is effective to lower the print permission rate of the ejection port positioned at the end compared with that of the ejection port positioned at the center as shown in FIG. 6 to reduce the “end-deviation.”
However, currently when reduction in the ejection volume is being further promoted, as the inventors carried out a diligent examination, they confirmed that only droplet ejected from the ejection port at the end are not always deflected toward the center.
FIG. 7 is a schematic view showing a deflection state in an ejecting direction different from the end-deviation. In FIG. 7, the reference numeral 81 denotes a printing head, and 256 ejection ports for ejecting ink droplets of 0.6 pl are arranged at pitches of 1200 dpi (intervals of approximately 21 μm). In FIG. 7, 16 every 16 ejection ports arranged per one line are shown as 1n, 17n to 241n, for convenience. The inventors printed an image 82 of 50% duty at a print density of 1200 dpi while making a carriage, on which such a printing head 81 is mounted, scan in relation to the print medium at 251 inch/sec. In this case, 100% duty shows a state where the dots are printed on all pixels arranged at 1200 dpi. The distance between the ejection port surface and the print medium in printing was 1.0 mm.
In FIG. 7, the reference numeral 83 denotes a dot formed by the ink droplets ejected from each of the ejection ports 1n, 17n to 241n on the print medium. According to the example, in an array of a series of ejection ports, although deflections of the ink ejected from the ejection ports positioned at the center and both ends are not found, ink ejected from the ejection ports positioned in the vicinity of the middle between the center and the ends is deflected in a ejection port arrangement direction (sub-scanning direction). The inventors confirmed that the deviation amount of the most deflected ink in the sub-scanning direction was approximately 15 μm.
FIG. 8 is a graph showing the relationship between the position of the ejection port and the print position deviation obtained in the mentioned examination. In FIG. 8, the horizontal axis shows an ejection port number representing the position of the ejection port, and the vertical axis shows a print position deviation amount of the dot printed with the ink droplets ejected from each ejection port in the sub-scanning direction. As shown in FIG. 8, in the example, the position of the dot printed via the ejection port positioned at the middle between the outermost ends and the center is most deviated in the array of the series of ejection ports. The print position deviation amount and the deviation direction are gradually fluctuated in accordance with the arrangement position of the ejection ports, and each is almost symmetrical in relation to the center of the ejection port array.
FIG. 9 is a view showing a print state in the case where an image is actually printed by one-pass with use of the printing head in such a print state. A printing head 101 moves from left to right in FIG. 9 at a predetermined speed with it mounted on the carriage, and ink is ejected from each ejection port 31 at a ejecting frequency corresponding to the moving speed. In FIG. 9, a white streak shown in FIG. 3 is not generated in an end area of an image formed by each print scanning. However, since an dot impact position via the ejection port positioned at the slightly inside from the end is deviated toward the center, a part having a high density 102 and a part having a low density 103 are alternately arranged.
Even if an image is formed by using the mask patterns (FIG. 6) disclosed in Japanese Patent Laid-Open No. 2002-096455 for the printing head in such a print state, the density unevenness is not reduced. If the mask patterns shown in FIG. 6 are used which extremely reduce the print permission rate of the end area having few deviations and raises the print permission rate of the area having many deviations, adverse effects due to the density unevenness becomes more conspicuous.